1. Field of the Invention
The present invention relates to an apparatus and method for sensing liquids, vapors and gases and to a current regulating device and, more particularly, to a method and apparatus for electrically sensing liquids, vapors and gases having virtually any Van der Waals' constant and to a nonlinear device for maintaining a relatively constant current through the device responsive to variations in voltage across the device.
2. Description of the Prior Art
It is an increasingly important task in any industrial society to detect the presence of the liquid, vaporous and gaseous substances. For example, detection may be of consequence since the substance may be intrinsically hazardous due to its explosive, flammable, toxic or noxious character. Obviously, detection is doubly important when such substances enter confined areas where they are unwanted, such as living spaces, mines, bilges, storage tanks, trailers aircraft and the like.
Detection may also be important where, although the substance is not particularly perilous in itself, its presence is an indication of some undesirable condition. For example, a hazardous fire may be detected by its early products of combustion which are not as hazardous as the fire itself. Similarly, the presence of a particular substance in the environment surrounding the detecting apparatus may indicate a leak in a supposedly tight system.
One detection devices used in the present invention is disclosed in U.S. Pat. No. 3,045,198, issued July 17, 1962, to Dolan et al. In its basic form, the detection device disclosed in the Dolan et al patent includes a layer of resilient material which is secured to a rigid base member. The active element, a stratum discrete, electrically conductive, adsorbent particles, adheres to the layer of resilient material, which serves to individually anchor each particle. A pair of spaced apart electrodes are in electrical contact with the stratum of conductive, adsorbent particles. Under reference conditions, as upon exposure to pure atmospheric air, the detection device will normalize and develop a characteristic resistance between the electrodes which is a function of the resistance of the stratum of conductive, adsorbent particles located between the electrodes. However, when the detection device is exposed to the liquid, vapor or gas being sensed, it is found that its resistance changes, usually by increasing.
What is believed to occur is that, in accordance with known principles, minute quantities of the substance being sensed are adsorbed onto the surface of each adsorbent particle, thereby forming a uniform, monomolecular layer which coats the surface thereof. The force of adsorption, known as the Van der Waals' adsorption force, is so great that the layer of adsorbed substance will actually interpose itself between adjacent adsorbent particles which are normally in contact and separate them. As a result, conduction paths established during normalization, when the detection device was exposed to a reference environment, are disrupted, and the characteristic resistance of the detection device is changed, thereby signaling the presence of the sensed substance. As noted in the reference patent, the changed resistance of the detection device is correlated to the Van der Waals' constant; and generally increases as the constant increases.
Of course, the concentration of the sensed substance to which the detection device is exposed has a bearing on the response time of the device; but given sufficient time, even extremely low concentrations of the sensed substance will be noticeably sensed. Upon return of the detection device to the reference environment, the layer of adsorbate gradually dissipates, returning the adsorbent particles to their normal conductive contact, and thus returning the device to its characteristic, normalized resistance.
Through extensive experimental it was discovered that the detection device disclosed in the referenced patent, while able to detect some substances, was insensitive to others. Through further study and experimentation, it was discovered that the detection device, when used in accordance with the teachings disclosed therein, was generally sensitive to those substances having a Van der Waals' constant which was greater than about 9, such as gasoline or diesel fuel, for example. However, its lack of sensitivity to certain other substances presented severe limitations on the usefulness of the detection device, inasmuch as a host of common substances have a Van der Waals' constant of about 9 or less. Thus, detection of carbon dioxide, carbon monoxide, propane, acetylene, natural gas and the like, all of which have a Van der Waals' constant of 9 or less, was impossible when using the detection device as taught in the Dolan et al patent.
Many years of research effort were spent attempting to modify the prior art detection device so that it would be able to detect such substances. If it were able to detect carbon dioxide and carbon monoxide it could be fabricated as a component in a fire detector and thus be useful to save both life and property. If it were able to detect natural gas, for example, it could be fabricated as a leak detector for such equipment as natural gas pipelines or LNG (liquid natural gas) transport ships. Of course, many other applications for a workable detection device able to sense substances having a Van der Waals' constant of less than 9 are readily apparent to those skilled in the art, and the uses mentioned are only by way of example.
In an effort to improve the prior art detection device and to make it sensitive to substances having a Van der Waals' constant of 9 or less, a multitude of approaches were tried. Varying the adsorbent particle size, composition, and mixture did not work. Selection of different material from which to fabricate the base member and resilient layer did not help. Changing the techniques by which the adsorbent particles were anchored to the resilient layer to thereby alter the depth and security with which each adsorbent particle was anchored also proved fruitless. Modifying the electrodes' composition and configuration was ineffectual. No matter what was tried, it was not possible to sense substances having a Van der Waals' constant of less than about 9.
However, through a fortuitous accident when the current-carrying capabilities of the prior art detection device were being measured it was noticed than an anomaly occurred at certain current and voltage levels. That is, as the voltage across the device was increased the current increased linearly in accordance with Ohms law until a specific value was reached. Thereafter the current remained relatively constant. Consistent with this finding, it was discovered that as the current through the detection device was increased, the voltage across the device also increased substantially in accordance with Ohm's Law until a specific value was reached. Thereafter, the voltage began to rise at a much faster rate than it would have as predicted by Ohm's Law.
Fortunately, while the device was being operated in this non-linear condition, (i.e., small changes in voltage across the device failed to bring substantially the changes in current through the device as predicted by Ohm's Law and very small changes in current through the device caused relatively large changes in voltage across the device) the device was tested to determine if it was still able to sense substances. Surprisingly, the new apparatus was able to detect not only those substances having a Van der Waals' constant of greater than about 9, but it was even able to detect those substances having a Van der Waals' constant of about 9 or less. A variety of liquids, vapors and gases were tested, and even helium, with a Van der Waals' constant of only 0.03412, was readily detectable. In each case, upon exposure to these substances, an easily detectable current change through the detection device occurred that was superimposed on the milliampere order of base current flowing therethrough when the device was powered by a constat voltage source, and an easily detectable voltage change across the device occurred when the device was powered by a constant current source.
The discovery that the prior art detection device, when operated in this non-linear condition, was sensitive to even those substances having a Van der Waals' constant of about 9 or less was doubly surprising since one of the prime benefits of the prior art device was that it was a "cold" sensor. That is, because it operated at the ambient temperature and employed no hot elements, it could be safely used to detect even explosive or flammable substances. Its cold operation was the result of the fact that its resistance changed upon exposure to the sensed substance and thus only a few microamperes were needed to detect this resistance change.
Operating the detection device in a non-linear manner was not an intuitive step to take for two reasons. First, non-linear operation is unnecessary when sensing substances by detecting resistance changes in the manner taught by the Dolan et al patent. Secondly, it would be thought that a high current level through the device might raise the temperature of the device to a dangerously hot level or cause ionization of the device which would render it inoperative.
Although it is not certain of the exact explanation as to the operation of his non-linear sensing apparatus, it is theorized that when an electrical current of sufficient quantity is caused to pass through the conductive, absorbent particles, the particles are heated to a temperature just slightly above the temperature of the environment to which the sensing apparatus is exposed. Then when the current saturated detection device is exposed to a substance with a Van der Waals' constant of about 9 or less, the sensed substance is adsorbed on the adsorbent particles and the heat of adsorbtion thereby released slightly raises the temperature of the particles still further so that a readily detected current change through or voltage change across the device occurs. However, it should be noted that due to its unusual property of operating in a non-linear manner at a mere milliampere order of base current, the detection device is still being operated as a "cold" type sensor, for such a minute current flow causes no substantial heating of the detection device as a whole.
It should be noted that there has been a change in the system of computing the Van der Waals' constant since the Dolan et al patent has issued, as is reflected in the current edition of the Handbook of Chemistry and Physics, published by the Chemical Rubber Company of Cleveland, Ohio. The current system is being used herein.
Non-linear devices such as zener diodes are employed in a variety of applications such as for voltage regulation. These devices are "current saturation" devices; that is, the current through the devices rises linearly with the voltage across the device to a certain point. As the current is increased further, the voltage remains relatively constant.
It is often desirable to regulate the current to an electronic device, but this can only be accomplished by relatively complex and expensive circuits or devices. Current regulation would be greatly simplified by a single device in which the current through the device remained relatively constant as the voltage across the device increased beyond a certain point. However, no device having this characteristic is known to exist.